Well cementing is a process used in penetrating subterranean zones (also known as subterranean formations) to recover subterranean resources such as gas, oil, minerals, and water. In well cementing, a well bore is drilled while a drilling fluid is circulated through the well bore. The circulation of the drilling fluid is then terminated, and a string of pipe, e.g., casing, is run in the well bore. The drilling fluid in the well bore is conditioned by circulating it downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. Next, primary cementing is typically performed whereby a slurry of cement in water is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the well bore and seal the annulus.
Low density or lightweight cement compositions are commonly used in wells that extend through weak subterranean formations to reduce the hydrostatic pressure exerted by the cement column on the weak formation. Conventional lightweight cement compositions are made by adding more water to reduce the slurry density. Other materials such as bentonite, diatomaceous earth, and sodium metasilicate may be added to prevent the solids in the slurry from separating when the water is added. U.S. Pat. No. 4,370,166 discloses this method for forming lightweight cement compositions. Unfortunately, this method has the drawback that the addition of more water increases the cure time and reduces the strength of the resulting cement.
Lightweight cement compositions containing hollow spherical beads have been developed as a better alternative to the cement compositions containing large quantities of water. The hollow spherical beads reduce the density of the cement composition such that less water is required to form the cement composition. The curing time of the cement composition is therefore reduced. Further, the resulting cement has superior mechanical properties as compared to cement formed by adding more water. For example, the tensile and compressive strengths of the cement are greater.
The hollow spherical beads are usually cenospheres or glass spheres. Cenospheres are hollow spheres primarily comprising silica (SiO2) and alumina (Al2O3) and are filled with gas. Cenospheres are a naturally occurring by-product of the burning process of a coal-fired power plant. Their size may vary from about 1 to 350 μm. Such hollow spherical beads suffer from the drawback of being fragile under pressure. Consequently, the hollow spherical beads tend to break when subjected to hydraulic pressures in excess of 1,000 psi. Unfortunately, hollow spherical beads mixed in a cement slurry can experience such pressures after placement of the slurry into a well bore. The collapse of the hollow spherical beads causes an increase in the density of the cement slurry. Therefore, the calculations of the quantity of slurry required to cement a given volume in the well bore are inaccurate because they are based on the density of the cement composition under atmospheric pressure (at the surface) rather than under the pressure existing downhole. To overcome this limitation, synthetic glass beads have been developed that have optimized wall thickness for withstanding the hydraulic pressure of the cement composition in the well bore. However, the cost of producing synthetic glass beads is very expensive. Another problem encountered when using a cement composition containing cenospheres is that the resulting cement exhibits high brittleness.
During the life of the well, the cement sheath is subjected to detrimental cyclical stresses due to pressure and/or temperature changes. A need therefore exists to develop a cement composition that can be used to form a less brittle cement having properties that enable it to withstand pressure and temperature fluctuations. It would also be desirable to maintain the density of the cement composition at a desired level after the cement composition is placed in a well bore, thereby ensuring that the surface calculations would be applicable to the downhole situation. The present invention advantageously utilizes a relatively inexpensive process to control the density of a cement composition containing spherical beads, even after those beads have collapsed under pressure. This process also improves the mechanical properties of the resulting cement, such as its brittleness, ductility, and resilience.